Aircraft battery systems

ABSTRACT

Aircraft with battery thermal regulation systems, including: an aircraft air inlet and an aircraft air outlet; a battery pack including battery cells; and a battery thermal regulation system including a first air channel, a surface of which is in thermal communication with the battery cells so that air flowing through the first air channel exchanges heat with the cells through the surface; and an airflow device for driving an airflow. The battery thermal regulation system has a first and a second mode operation. In the first mode, the first air channel is fluidly connected with the aircraft air inlet and the aircraft air outlet whereby, during flight of the aircraft, air can enter the aircraft through the air inlet, flow through the first air channel and exit the aircraft through the air outlet. In the second mode, the airflow device drives an airflow through the first air channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority fromUnited Kingdom Patent Application No. 2108576.6, filed on 16 Jun. 2021,the entire contents of which are incorporated herein.

TECHNICAL FIELD

The present disclosure relates to batteries and battery thermalregulation systems, particularly but not exclusively for use in electricand hybrid electric aircraft.

BACKGROUND

Interest in aircraft with electric and hybrid electric propulsionsystems is increasing because of the need to reduce carbon emissions andpollution, and because of developments in the facilitating electricaltechnologies. Hybrid electric propulsion systems include both internalcombustion engines, for example gas turbines or diesel engines, andenergy storage, typically in the form of batteries. Purely electricpropulsion systems completely dispense with internal combustion enginesand use only batteries or, in some instances, fuel cells, as an energysource for their propulsors.

Urban Air Mobility (UAM) refers to the use of aircraft—typicallyelectric and hybrid electric aircraft—to transport passengers relativelyshort distances, for example tens or perhaps hundreds of kilometres.Most proposed UAM platforms, sometimes known as ‘air taxis’ or ‘flyingtaxis’, have Vertical Take-Off and Landing (VTOL) or Short Take-Off andLanding (STOL) capabilities so that the aircraft can take-off and landat locations convenient for passengers, for example at so-called‘vertiports’ close to or in urban environments. It is expected that thenumber of passengers carried by UAM platforms will be relativelysmall—likely fewer than twenty and typically of the order of five toten.

Various configurations have been proposed for UAM platforms, and oneexample is illustrated in FIG. 1 . The aircraft 10 includes a fuselage11 and a distributed propulsion system which includes a number, in thiscase six, of propulsors 12 in the form of open rotors driven by rotaryelectric machines. The propulsors, which in this case are attached towings 13 or other flight surfaces 14, are tiltable between a VTOLconfiguration (shown FIG. 1 ) and a horizontal flight configuration tofacilitate both modes of flight. Other proposed configurations arehelicopter-like configurations, for example multi-copters (e.g.,quadcopters with ducted fans) and the like.

SUMMARY

The present invention relates to the thermal regulation of battery packsof aircraft.

According to a first aspect, there is provided an aircraft comprising:an aircraft air inlet and an aircraft air outlet; a battery packcomprising battery cells; and a battery thermal regulation system. Thebattery thermal regulation system comprises: an airflow device fordriving an airflow; and a first air channel, a surface of which is inthermal communication with the battery cells so that air flowing throughthe first air channel exchanges heat with the cells through the surface.The battery thermal regulation system has a first mode of operation anda second mode operation. In the first mode of operation, the first airchannel is fluidly connected with the aircraft air inlet and theaircraft air outlet whereby, during flight of the aircraft, air canenter the aircraft through the air inlet, flow through the first airchannel and exit the aircraft through the air outlet. In the second modeof operation, the airflow device drives an airflow through the first airchannel.

An arrangement that leverages the flow of air provided by the aircraft'smovement to cool the battery cells may reduce or eliminate the weight,complexity and maintenance associated with liquid cooling systems. Sucha cooling system may however leave the aircraft without means forthermally regulating the cells when there is no or little airflow, forexample when the aircraft is on the ground or if the air inlet becomesblocked during flight. The provision of a second mode of operation inwhich airflow is driven by a separate airflow device, such as a fan, canaddress this and may, for example, permit more rapid charging of thebattery pack between flights.

The battery thermal regulation system may further comprise a second airchannel and the airflow device may be arranged in the second airchannel. In the second mode of operation, the first air channel may bein fluid communication with the second air channel and the airflowdevice may drive an airflow through the second air channel and throughthe first air channel. Providing the airflow device in a second channelmay reduce the airflow device's impact during the first mode ofoperation and improve ease of maintenance and assembly.

In the second mode of operation, the first air channel may bedisconnected from the aircraft air inlet and the aircraft air outlet andthe first and second air channels form a closed loop. The use of aclosed loop may eliminate the need to rely on an open external inlet andoutlet for the supply of air.

The thermal regulation system may be a reconfigurable system having afirst configuration corresponding to the first mode of operation and asecond mode of operation corresponding to the second mode of operation.

The aircraft may further comprise one or more actuators fordisconnecting the first air channel from the aircraft air inlet and theaircraft air outlet and for connecting the first air channel to thesecond air channel.

The airflow device may be reversible, whereby the direction of theairflow driven through the first air channel by the airflow device isreversible. The aircraft may further comprise a controller configured tocontrol the reversible airflow device so that, in the second mode ofoperation, the direction of the airflow through the first air channel isreversed, e.g., periodically reversed. Reversing the direction of theairflow may prevent or reduce thermal gradients along the first airchannel.

The battery thermal regulation system may further comprise a cooler forcooling the airflow driven by the airflow device through the firstchannel. Cooling the airflow can improve the efficiency with which heatcan be removed from the cells.

The battery thermal regulation system may further comprise a heater forheating the airflow driven by the airflow device through the firstchannel. In this way the second mode of operation can be used to heatthe battery cells to bring them to a preferred range of operatingtemperatures, for example following a cold soak.

The aircraft may further comprise an adjustable airflow regulator forregulating the airflow through the first air channel. Thus, in the firstand/or second modes of operation, the airflow may be varied according topresent cooling/heating requirements.

The air channel may comprise a plurality of parallel air channelsseparated by dividing walls. There may be five or more parallel airchannels, for example between five and fifteen channels.

The aircraft may further comprise one or more additional channelsarranged to carry a flow of liquid in a closed loop. A portion of theloop may be in thermal communication with the battery cells so thatliquid flowing through the one or more additional channels exchangesheat with the cells. The liquid carrying channels may providesupplemental or replacement thermal regulation in the first and/orsecond mode. For example, liquid cooling may be used in addition to aircooling in the second mode of operation to allow rapid charging.

A portion of the loop of the one or more additional channels may followa serpentine path. The use of a tortuous path may increase the rate ofheat transfer to or from the cells.

The one or more additional channels may be arranged so that the one ormore additional channels form a non-zero angle with the horizontal. Theangle may be at least 1°, preferably at least 2°, more preferably atleast 5°. The angle may assist system degassing. The angle may be theangle formed when the aircraft is stationary on the ground or when theaircraft is in horizontal flight.

The first air channel, or each of the plurality of parallel air channelsof the of the first air channel, may have a larger cross-sectional areathan each of the one or more additional channels. Smaller liquidchannels may be easier to pressurize and drain.

The aircraft may further comprise means for filling and/or draining theone or more additional channels of liquid. In this way, if the liquidcooling is used only for ground-based charging, the liquid may bedrained prior to flight to reduce weight.

The air inlet and the air outlet may be: located in a fuselage of theaircraft; located in a propulsor of the aircraft; or located elsewhereon the aircraft. Placement within the fuselage may provide for betterdynamics and ease of assembly and maintenance. Placement within apropulsor may reduce the risk of fire or smoke spreading to the cabinand/or a higher airflow rate.

The first air channel may be an integral part of the battery pack. Thesecond air channel may be located within the aircraft outside of thebattery pack.

The battery pack may comprise a first array of battery cells; and asecond array of battery cells. The air channel may be arranged between(e.g., sandwiched between) the first and second arrays of battery cells,so that the two arrays are on opposite sides of the air channel.Portions of the surface of the air channel may be in thermal contactwith both the first and the second arrays of cells, whereby air flowingthrough the air channel exchanges heat with both the first and secondarrays of battery cells through the surface of the air channel. In sucha configuration opposite sides of the air channel are used for cooling,which reduces duplication of features and allows for an increased packenergy storage density.

The air channel may extend substantially the entire length of thebattery pack. The length of the battery pack may be measured parallel tothe general direction of airflow through the air channel, which maycorrespond to the general direction of horizontal flight of theaircraft. The battery pack may have a generally cuboidal shape, with alength measured parallel to the general direction of airflow through thechannel, a height generally parallel to gravity and perpendicular to thelength, and a width mutually perpendicular to the length and height.

The aircraft may be a purely electric aircraft, or it may be a hybridelectric aircraft.

In other embodiments the aircraft is a ‘more electric’ aircraft, inwhich propulsive thrust is exclusively or almost exclusively provided byone or more engines, and the battery is used to improve engineoperability, for example by adding or removing power from engine spoolsduring transients, or by powering electrical loads during enginetransients.

The aircraft may be of the VTOL type. For example, the aircraft may havea reconfigurable propulsion system reconfigurable between a verticalflight configuration and a horizontal flight configuration.

In other examples the aircraft may be of the STOL type.

According to a second aspect, the battery thermal regulation system ofthe first aspect is provided separately.

According to a third aspect, there is provided the combination of thebattery pack and the thermal regulation system of the first aspect.

The skilled person will appreciate that except where mutually exclusive,a feature described in relation to any one of the above aspects may beapplied mutatis mutandis to any other aspect. Furthermore except wheremutually exclusive any feature described herein may be applied to anyaspect and/or combined with any other feature described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with referenceto the accompanying drawings, which are purely schematic and not toscale, and in which:

FIG. 1 is a perspective view of an electric aircraft in a VTOLconfiguration;

FIG. 2 is a schematic illustration of an electric aircraft with abattery pack cooled by indirect air cooling;

FIG. 3 is a side-view of the aircraft of FIG. 1 in a horizontal flightconfiguration, further showing a possible location for a battery packwithin the fuselage of the aircraft;

FIG. 4 is a side-view of a propulsor of an aircraft incorporating abattery pack;

FIG. 5A is a perspective view of a battery pack with integral airchannels for indirect air cooling;

FIG. 5B is a perspective view of the battery pack of FIG. 5A with one ofthe caps removed;

FIG. 6A is a front view of the battery pack;

FIG. 6B is a cross-sectional front view of the battery pack, showingfurther detail including the locations of the battery cells;

FIG. 7 is a cross-sectional front view of the battery pack, illustratinghow thermal products vented by the cells may be removed from theaircraft;

FIG. 8 is a cross-sectional side view of another embodiment of thebattery pack, illustrating how thermal products vented by the cells maybe removed from the aircraft;

FIG. 9A is a schematic illustration of an electric aircraft with abattery pack cooled by indirect air cooling, connected to an externalair supply;

FIG. 9B is a flow diagram illustrating a method for thermally regulatinga battery of an aircraft; and

FIG. 10 is schematic illustration of an electric aircraft with areconfigurable battery thermal regulation system.

DETAILED DESCRIPTION FIG. 1

FIG. 1 illustrates an electric aircraft 10 which may be used for UAMapplications. The aircraft includes a fuselage 11, which incorporates acabin for occupants, and a distributed propulsion system which in thiscase has six open rotor propulsors 12 driven by rotary electricmachines. Also visible in FIG. 1 is a retractable undercarriage 15 inwhich a landing platform, in this case having wheels, can be stowedduring flight.

The size of the fuselage 11 and the cabin will depend on the applicationrequirements, but in this example they are sized for five occupantsincluding a pilot. It is however envisaged that some UAM platforms willnot require a pilot and will instead be flown under the control of anautopilot system.

Four of the propulsors 12 are attached to the wings 13 of the aircraft10, and the remaining two propulsors 12 are attached to a separateflight control surface 14 located towards the rear of the aircraft 10.In this embodiment the wings 13 and the rear control surface 14 aretiltable between a VTOL configuration (shown in FIG. 1 ) in which theaxes of the rotors point upward and a horizontal flight configuration(shown in FIG. 3 , with the front rotors omitted for clarity) in whichthe axes of the rotors point forward. The horizontal flightconfiguration, whilst principally used for horizontal flight, may alsobe used for taxiing and possibly STOL operation if supported.

The electrical systems, including the electric machines that power theaircraft 10, receive electrical power from one or more battery packslocated within the aircraft. The battery packs may be located in anysuitable part or parts of the aircraft, including the fuselage 11, thewings 13 and the propulsors 12.

Whilst the illustrated aircraft 10 is an electric VTOL (eVTOL) aircraft,it will be appreciated that UAM platforms could also be of the STOL typeand could also be hybrid electric aircraft that include both internalcombustion engines and batteries. Hybrid electric platforms may utilizesimilar distributed propulsion system configurations, but the underlyingpower system may be a series hybrid, parallel hybrid, turboelectric orother type of hybrid power system.

FIG. 2

The cells of the battery packs will require thermal regulation tomaintain optimal performance and to reduce the risk of overheating,which could result in damage to the cells or even fire. To this end, abattery pack and/or the aircraft 10 in which the battery pack isincorporated will include a battery thermal regulation system.

FIG. 2 illustrates an aircraft 10 in schematic form with a battery pack20 incorporated into the aircraft in accordance with some embodiments ofthe present disclosure. The battery pack 20 includes an integral airchannel 21 which passes through the battery pack. One end of the channel21 is fluidly coupled to an aircraft air inlet 16, and the other end ofthe channel 21 is fluidly coupled to an aircraft air outlet 17. Duringflight, the direction of which is illustrated by the arrow D′ in FIG. 2, ambient air enters the air inlet 16, flows through the channel 21 andflows out of the air outlet 17.

The wall 211 of the air channel 21 is in thermal contact with the cellsof the battery pack, so heat flows from (or to) the cells to (or from)the air flowing through the channel 21. Thus, the flow of ambient airresulting from the normal movement of the aircraft 10 is leveraged tocool (or heat) the cells.

It should be appreciated that although the air flowing through thechannel 21 is in thermal contact with the cells, the air does notdirectly contact the cells. Instead, the cells are enclosed within ahousing and exchange heat with the air indirectly through the wall 211of the channel 21. The cells may, for example, be attached to theexterior surface of the wall 211 of the air channel 21 (such that thewall 211 is a heat sink for the cells), or may be attached to a separateheat sink (e.g., an aluminium base plate) which is in close thermalcontact with the wall 211 of the air channel 21. Thus, although theambient air flow is used to cool (or heat) the cells, the air need notbe filtered or otherwise conditioned to remove moisture, particulates orchemicals which could damage or degrade the cells or electricalconnections of the battery pack. Nevertheless a filter may be providedto prevent debris entering and blocking the air channel 21.

Furthermore, provided the heat sink (e.g., the wall 211 of the airchannel 21 or a base plate coupled thereto) is made of a material with alow thermal mass (e.g., aluminium, magnesium or an alloy of magnesium),the temperature of the heat sink will remain approximately uniform overits entire length. The cells will therefore be cooled (or heated) in asubstantially uniform manner. In contrast this may not be the case insystems in which air flows in direct contact with the cells, due in partto unpredictable airflow and the change in temperature of the air as itflows through the pack.

The arrangement of FIG. 2 further includes an adjustable airflowregulator 18 located between the air inlet 16 and the entrance to theair channel 21 of the battery pack. The adjustable airflow regulator 18,which can take any suitable form such as vanes, an iris or a butterflyvalve, any of which may be electrically or mechanically actuated, allowsthe rate of the airflow through the channel 21 to be regulated inaccordance with the cooling or heating requirements of the pack 20. Inthis embodiment a further adjustable airflow regulator 19 is locatedbetween the exit of the air channel 21 and the air outlet 17.

In some embodiments, the airflow regulator 18 (and optionally theairflow regulator 19) can be adjusted to completely or almost completelyprevent the flow of air through the channel 21. This may be useful whereit is desirable to heat the cells, for example where the aircraft 10 hasbeen exposed to cold ambient conditions between uses or during use, asthe heat generated by the cells during their discharge is not carriedaway by airflow through the channel 21. The channel 21 may also becompletely or partially closed in some modes of operation of areconfigurable thermal management system, such as the one describedbelow with reference to FIG. 10 .

FIG. 3

FIG. 3 illustrates how the battery pack 20 of FIG. 2 may be locatedwithin the fuselage 11 of an aircraft 10.

In this embodiment the battery pack 20 is located aft of the cabin,which may be preferable to reduce the risk of gas, smoke or fire fromthe batteries reaching the passenger cabin. Whilst aft the cabin, thebattery pack 20 is located in a middle region of the fuselage 11 suchthat, when the pack is installed, the centre of gravity of the aircraft10 coincides with the battery pack 20. The battery pack 20 is relativelyheavy and dense, so locating the pack so that it coincides with thecentre of gravity of the aircraft helps reduce its impact on thedynamics of the aircraft 10.

So that air can enter the air channel 21 of the battery pack 20, thefuselage 11 includes an air inlet 16 in the form of an opening oraperture in an external surface of the fuselage. Likewise, the fuselage11 includes an opening or aperture in an external surface to form an airoutlet 17. The size and location of the apertures 16, 17 should beselected to provide adequate airflow during horizontal flight and,preferably, also during vertical take-off and landing. In someembodiments there may be multiple air inlet openings 16 in fluidcommunication with the air channel 21 so that there is adequate airflowduring both horizontal flight and vertical movement. The air outlet 17may also include multiple openings.

The path between the opening that forms the air inlet 16 and theentrance to the air channel 21 can take any form suitable for thespecific application. In some embodiments, the opening and path betweenthe opening and the air channel 21 may take the form of a continuousopening and channel defined in the airframe. In other embodiments, aseparate air conduit may be attached between the opening and theentrance of the air channel 21. The path between the opening that formsthe air outlet 17 and the exit of the air channel 21 can take a similarform.

It should be appreciated that although only one battery pack 20 isillustrated, the aircraft 10 and its fuselage 11 may include multiplebattery packs 20. Multiple battery packs 20 may be arranged in parallelairflow, for example.

FIG. 4

FIG. 4 illustrates how the battery pack 20 of FIG. 2 may be locatedwithin a propulsor 12 of an aircraft 10.

As can be seen, the battery pack 20 is located within a housing 121 ofthe propulsor located aft of the rotor 122. An air inlet 16 throughwhich air can enter the housing 121 and reach the air channel 21 of thebattery pack 20 is located at the front of the housing 121, in this casebeneath the rotor 122. The air then exits the air channel 21 andsubsequently the housing 121 through an air outlet 17.

Locating battery packs 20 within propulsors 12 may advantageously allowa battery, motor and power electronics combination to be assembledtogether as a single unit. Further, it increases the distance betweenthe battery and cabin, which may reduce some of the risks associatedwith battery fire and other faults. Conversely, it may make ground-basedmaintenance of the battery packs more difficult and make the propulsorsbulkier.

FIG. 5A

FIG. 5A is a perspective view of a battery pack 20 of the type describedabove with reference to FIGS. 2-4 .

The battery pack 20 includes a central, straight, longitudinallyextending air channel 21 of rectangular cross-section. As can be seen,the air channel 21 actually comprises multiple (in this case ten)parallel air channels separated by dividing walls. The air channel 21extends the entire length of the pack 20, which in a specific embodimentis approximately 1 metre long.

Either side of the air channel 21, the pack 20 includes a housingportion 22 a, 22 b which houses an array of battery cells, and aremovable module cap 23 a, 23 b.

FIG. 5B

FIG. 5B shows the battery pack 20 with one of the module caps 23 aremoved. A portion of one of the housing portions 22 a is also omittedto show some of the battery cells 200 of the pack 20.

The cells of the battery pack 20 may be of any suitable type, forexample cylindrical cells, prismatic cells, pouch cells or cells ofanother type. In one embodiment the cells are cylindrical cells, forexample 18650 cylindrical cells, which extend between opposed first andsecond ends.

FIG. 6A

FIG. 6A is a front-on view of the battery pack 20 of FIGS. 5A-B.

From FIG. 6A it can be more clearly seen that the air channel 21includes multiple (in this case ten) parallel air channels 210 separatedby vertical dividing walls 212. In preferred embodiments the entire airchannel 21, including the dividing walls, is integrally formed as thisimproves the mechanical stiffness of the channel 21, which forms thestructural backbone of the battery pack 20. In this embodiment the airchannel 21, and the multiple parallel channels 210 that form the airchannel 21, are of rectangular cross-section but it will be appreciatedthat other shapes may be used.

The aspect ratios of the parallel channels 210—defined as their height(h) divided by their widths (w)—is preferably greater than 1 as thisprovides greater vertical stiffness and reduces deflection of thestructure under load. This can decrease the fatigue stress to which thewire bonds and other electrical connections in the pack 20 may besubject. An aspect ratio h/w of between 1.1 and 1.6, between 1.2 and1.5, or between 1.25 and 1.45 may provide particularly good properties.In the illustrated embodiment the two outer-most channels have an aspectratio of about 1.32, and the remaining eight channels have an aspectratio of about 1.43.

Whilst not shown in the embodiment of FIG. 6A, in some embodiments theaircraft 10 and/or battery pack 20 may include a secondary thermalregulation system to provide supplemental or replacement thermalregulation during flight and/or during ground-based operations such ascharging and pre-heat/pre-cool. For example, a closed loop liquidcarrying system may flow a supply of liquid in thermal contact with thecells 200 through one or more additional channels in the battery pack20.

The one or more additional channels, which may be of smallercross-section than the air channels 210, may be nestled between adjacentparallel air channels 210 so as to cool (or heat) the cells through thesurfaces 211 a, 211 b. The portion of the flow path of the additionalchannels that is in thermal contact with the cells may also follow atortuous, serpentine path to maximize the heat transfer.

FIG. 6B

FIG. 6B shows the battery pack 20 in cross-section so that internaldetail is visible. The battery cells are omitted from FIG. 6B forclarity.

The battery pack 20 may be considered to include two sub-modules: onesub-module located on a first side of the air channel 21 and the secondsub-module located on the other, opposite, side of the air channel 21.Each of the two sub-modules houses an array of battery cells within theinternal volume of a housing portion 22 a, 22 b.

The cells of each sub-module are electrically connected together inseries and parallel (by connecting the cells to bus bars by wire bonds,for example) to increase the sub-module terminal voltage and the maximumrated discharge rate of the sub-module. The two sub-modules may then beconnected together in series or parallel to double the pack terminalvoltage or maximum discharge rate, or alternatively may not be connectedand may instead provide two independent power channels if this isdesired.

The internal volume of each housing portion 22 a, 22 b (hereafterreferred to as the housing portion 22) is divided into chambers 220 bydividing walls 221, with each chamber 220 housing one or more cells. Theuse of chambers 220 and dividing walls 221 improves the mechanicalstiffness of the battery pack 20 and promotes thermal isolation betweencells, which limits the spread of thermal events.

The cells of each sub-module are cooled from one end (the end closest tothe air channel 21) by the air passing through the air channel 21.Specifically, one end of the cells of the first sub-module are inthermal contact with a surface 211 a of the wall 211 of the air channel21, and exchange heat with the air through the surface 211 a of thewall. Similarly, one end of the cells of the second sub-module are inthermal contact with an opposite surface 211 b of the wall 211 of theair channel 21, and exchange heat with the air through the surface 211 bof the wall 211. The use of opposite sides 211 a, 211 b of the wall 211to cool separate sub-modules improves the overall energy density of thebattery pack 20.

In the illustrated embodiment the cells are retained within siliconerubber bushings 222 which are attached to, or are in close thermalcontact with, an external surface of the wall 211 of the air channel.Thus, heat is exchanged between the cells and the air passing throughthe air channels 21 via the silicone rubber bushings 222 and the wall211 of the air channel. In other embodiments, the silicone rubberbushings 222 are attached to a base plate, and the base plate isattached to or at least in close thermal contact with the externalsurface of the wall 211 of the air channel 21. As well as providingthermal bridging, the bushings 222 hold the cells under compression toprovide mechanical support and provide electrical insulation between thecells and the heat sink. Thermal grease may be used to aid the insertionof the cells into the bushings 222, which allows for a tighter fit andthus greater compression and cell retention.

In preferred embodiments the housing portions 22 a, 22 b (including thedividing walls 221 if present) and the air channel 21 (including thedividing walls 212 if present) are integrally formed. In this way, thecombination of the air channel 21 and the housing portions 22 a, 22 bform the main structural component of the battery pack and provide goodmechanical stiffness. The air channel 21 and/or the housing may beformed from aluminium, magnesium, a magnesium alloy or another materialwith a low thermal mass.

The integrally formed air channel 21 and/or housing portions 22 arepreferably formed by extrusion. The use of extrusion not only improvesthe mechanical strength of the pack 20 by eliminating the need forinternal joining, but also provides good dimensional tolerances andallows for the addition of complex design features. For instance, theinternal surface of the wall 211 of the air channel 21 may includefillets, or complex fin designs (e.g., tapering from the wall 211 to atip), and extrusion may allow for the inclusion of such design featureswith minimal additional cost. It will however be understood that thepack 20 may be fabricated in other ways, for example by machining oradditive layer manufacturing (ALM).

Each of the sub-modules of the battery pack further includes a cap 23 a,23 b (hereafter cap 23) which closes the internal volumes of the housingportions 22 a, 22 b. As well as generally sealing the battery pack 20 toprotect the cells from the external environment, each cap 23 defines aninternal volume which provides the respective sub-module with a cellventing region 230. During use, cells may overheat or otherwiseexperience faults which cause them to vent gas, flames, smoke and otherparticulates. The cell venting region 230 within the cap 23 provides aspace into which the cells can vent these thermal products in acontrolled manner, away from the core of the battery pack 20 so as toavoid damage to cells and propagation of thermal events between cells.

Generally, cells have a pre-defined venting direction, which in the caseof cylindrical cells is from one of the two ends. The cells arepreferably oriented within the housing so that their venting ends pointinto the venting region 230. In the illustrated embodiment, an internalsurface 231 of the module cap 23 is provided with holes or apertures sothat the cells can vent directly into the venting region 230.

FIG. 7

FIG. 7 illustrates a first embodiment of a battery pack 20 in which apath is provided between the venting region 230 and the exterior of theaircraft 10. Providing such a path helps prevent vented products,particularly gas and smoke, from accumulating within the battery pack 20and the aircraft 10.

In this embodiment, each of the sub-modules includes a cap 23 in whichthere is a venting region 230. The cells 200 a, 200 b of the sub-modules(hereafter the cells 200) are oriented within the housings 22 so thattheir venting directions are directed towards a venting region 230. Aninternal surface 231 of the cap 23 has apertures through which theventing ends of the cells can vent into the venting region 230.

So that vented thermal products can escape the battery pack 20 andultimately the aircraft 10, the battery pack 20 further includes aninternal volume or channel 232 which provides a path (indicated by thearrows) between the venting region 231 and an air channel 213. The airchannel 213 is, like the air channels 210 described above, in fluidcommunication with the air inlet 16 and the air outlet 17. Thus, thermalproducts entering the air channel 213 from the venting region 231 viathe path 232 are swept away by the flow of air that results from themovement of the aircraft 10.

In the illustrated embodiment the air channel 213 is dedicated to theremoval of vented products from the battery pack 20. In other words, thechannel 213 is not one of the cooling air channels 210 describedpreviously, but an additional channel that does not serve a coolingfunction. Whilst alternative embodiments may utilize one or more of thecooling air channels 210 to vent thermal products, providing one or moreseparate, dedicated channels 213 for this purpose may be preferable. Insome embodiments described herein the cooling air channels 210 may beblocked or reconfigured into a closed loop in some modes of operation,particularly ground-based operations such as recharging. In theseinstances, the provision of a separate channel 213 prevents ventedproducts from being trapped in the air channels 210 and possibly beingrecirculated in a closed loop. Further, the vented products willgenerally be hot, so venting into the cooling channels 210 could reducethe cooling efficiency.

The channel 232 between the venting region and the air channel 213 ispreferably located towards an exterior region of the battery pack. Thishelps limit the transfer of heat from the vented products to the cells200.

The battery pack 20 may further include a pressure-sensitive element 233which is configured to seal the venting region 230 until the pressure inthe venting region exceeds a threshold pressure. Specifically, theventing region 230 is ordinarily sealed, but if vented productsaccumulate in the venting region 230 to the point that the internalpressure reaches a threshold level, the pressure-sensitive element 233allows the passage of the vented products through the path 232 to theair channel 213. The pressure-sensitive element can take any suitableform, but examples include a sacrificial burst disk made from analuminium sheet or other suitable material; and a one way valve, forexample a sprung metal valve.

Although not illustrated, some or all of the path between the ventingregion 230 and the exterior of the aircraft 10 may be filled with aflame arrestor to reduce the risk of the vented products igniting a fireor explosion. The flame arrestor can be included in any portion of thepath, including one or more of the venting region 230, the air channel213 and the channel 232 between the venting region and the air channel.213. Examples of suitable flame arrestors include metal foams, metalmesh and ceramic mesh.

FIG. 8

FIG. 8 illustrates a second embodiment of a battery pack 20 in which apath is provided so that vented products can escape to the exterior ofthe aircraft 10. The battery pack 20 is shown in a side-on view in whichair passes through the central air channel 21 from left to right.

In this embodiment, in addition to the central cooling air channel 21,the battery pack includes further air channels 235 a, 235 b into whichthe cells 200 vent directly. Like the central air channel 21, the airchannels 235 a, 235 b are in fluid communication with an air inlet andthe air outlet so that the movement of the aircraft creates a flow ofair through the channels 235 a, 235 b. This flow of air sweeps thevented products away out of the battery pack 20 and out of the aircraft10. In some embodiments the further air channels 235 a, 235 b supplementthe cooling provided by the main, central cooling channel 21.

Thus, unlike the embodiment of FIG. 7 in which there is path 232 betweena venting region 230 and air channel 233, in the embodiment of FIG. 8the venting region is an air channel 235 and the cells vent directlyinto the air channel 235. This reduces the amount of time the ventedproducts spend within the battery pack 20, which reduces the risk offire and explosion and the propagation of thermal events.

The air inlet(s) and air outlet(s) to which the further air channel(s)235 a, 235 b are coupled may be the same air inlet(s) 16 and airoutlet(s) 17 that serve the main air channel 21. However, in someembodiments one or more additional air inlets or outlets may be providedto serve the further air channels 235 a, 235 b. This may be preferable,for example to ensure thermal products can escape the battery pack 20and aircraft 10 during recharging where the main air inlet 16 and outlet17 may be blocked or otherwise unavailable. Furthermore, while in theillustrated embodiment the air channels 235 a, 235 b are a part of thebattery pack 20, in other embodiments the air channels 235 a, 235 b maybe located within the aircraft 10 outside of the battery pack 20.

Whilst the embodiments of both FIG. 7 and FIG. 8 utilize air channels213, 235 to leverage an airflow to remove vented products from theaircraft 10, other embodiments may take different approaches. Forexample, in another group of embodiments, a path (e.g., a tube, channelor other form of conduit) is provided between a venting region and anopening or aperture in the external surface of the aircraft 10. In suchembodiments it would be preferable to select the locations of thebattery pack 20 and the opening in the aircraft 10 so as to keep thelength of the path to a short distance. For example, it would bepreferable to keep the length of the path below 1 metre, perhaps evenless than 50 cm or 30 cm. This may reduce the likelihood and possibleimpact of ignition of the vented products as they move from the ventingregion to the exterior of the aircraft.

FIG. 9A

As explained previously, one of the potential advantages associated withusing the ambient airflow to thermally regulate the cells 200 is weightreduction. This is because some of the equipment typically associatedwith on-board cooling systems, for example pumps, may be omitted, or maybe of reduced size. However, this may leave the aircraft 10 with reducedability to thermally regulate the cells 200 when there is little or noambient airflow through the channel 21. This may be a limitation whenthe aircraft 10 is stationary on the ground and the battery pack 20 isbeing charged, as the temperature of the cells 200 can limit the rate atwhich they can be safely charged.

Thus, in accordance with some aspects, the air inlet 16 of the aircraft10 may be connected to an external supply of air 30 so that air can bedelivered through the air channel 21 whilst the aircraft 10 is on theground. This is illustrated in FIG. 9A. The supply of air 30 may, forexample, be a ground-based device such as a fan which can be stored atan airport, vertiport or the like and connected to the air inlet 16 asrequired.

In order to further improve the efficiency with which heat can beremoved during charging, the air supply 30 may be a refrigerated airsupply. The air supply 30 may, for example, incorporate a cooler (whichmay take the form of a heat exchanger) so that the air forced throughthe air channel 21 is cooler than the ambient air temperature.

In other examples the supply of air 30 may not be cooled (i.e. ambientair may be used), or the air may even be heated by a heater incorporatedinto the supply 30. This allows the cells 200 to be heated by theairflow. Pre-heating the battery cells 200 to a preferred operatingtemperature prior to flight or prior to charging reduces the risk ofdamage to the cells and may improve the efficiency of operation.

In some embodiments, a closed loop may be formed by connecting the airsupply 30 to both the inlet 16 and outlet 17 of the aircraft. As well asimproving the efficiency of the system, recirculating the same air mayreduce the need to filter the air to prevent debris and particlesblocking or becoming entrained in the air channel 21. The inlet 16and/or outlet 17 may be equipped with connectors to facilitateconnection to the supply 30.

Utilizing a closed loop also allows the direction of airflow through theair channel 21 to be reversed. In other words, the direction of airflowmay be reversed so that, instead of flowing from the inlet 16 to theoutlet 17, air flows from the outlet 17 to the inlet 16. Reversing thedirection of airflow may reduce the tendency for thermal gradients todevelop along the length of the air channel 21. In some embodiments acontroller, which may be integrated into the air supply 30 or theaircraft 10, may be provided and configured to control and periodicallyreverse the direction of air flow.

Whilst not illustrated in FIG. 9A, the aircraft 10 and battery pack 20may also include a secondary thermal regulation system in the form ofone or more additional, liquid carrying, channels, as described abovewith reference to FIG. 6A.

FIG. 9B

FIG. 9B is a flow diagram illustrating a method of thermally regulatinga battery pack 20 of an electric or hybrid electric aircraft 10. Itshould be appreciated that, unless the context clearly dictatesotherwise, the steps S1-S6 of the method can be performed in a differentorder.

At step S1, an external, ground-based supply of air 30 is connected toan air inlet 16 of the aircraft. For example, a fan or other source ofpressurized air is connected to the inlet using a flexible tube or otherconduit so that the air supply 30 can force air through the tube intothe air inlet 16. In some embodiments the tube and inlet 16 may beequipped with complimentary connectors (e.g., male and femaleconnectors) to improve the ease with which the air supply 30 can beconnected to the inlet 16.

At step S2, the supply of air 30 is further connected to an air outlet17 of the aircraft 10 so as to form a closed loop. For example, on theside of the fan opposite to the air inlet 16, the air supply may beconnected by a tube or other conduit to the air outlet 17 of theaircraft. In this way, air driven through the air inlet 16, through theair channel 21 and out of the air outlet 17 is returned to the airsupply 30 for recirculation. It will be appreciated that, in someembodiments, step S2 is not performed and an open loop is instead usedfor the remaining steps of the method.

At step S3, an external supply of electrical power is connected to thecells 200 of the battery pack 20 in order to charge the cells. Anysuitable source of electrical power can be used, for example mains poweror a portable source of power such as a battery or generator. It will beappreciated that step S3 may be omitted where the cells 200 do notrequire charging, for example where the cells are already charged and apre-heat operation is to be performed.

At step S4, the external air supply 30 delivers a flow of air throughthe air inlet 16, through the air channel 21 and out of the air outlet17. Where step S2 is performed, the air passing out of the air outlet 17returns to the air supply 30 for re-pressurization and recirculation.Otherwise, the air passing out of the air outlet 17 is discharged to theambient air.

Where the temperature of the air driven through the air channel 21 bythe air supply 30 is cooler than the temperature of the surface 211 ofthe air channel 21, the flow of air carries heat away and thus cools thecells 200. This is desirable where the cells are being charged (step S3)and may allow for more rapid charging. In order to permit even morerapid charging, the air supplied by the air supply 30 may berefrigerated to enhance the cooling rate.

Where the temperature of the air driven through the air channel 21 bythe air supply 30 is hotter than the temperature of the surface 211 ofthe air channel 21, the flow of air heat the cells 200. This isdesirable where the cells are being pre-heated to bring them up to apreferred temperature for operation (e.g., prior to flight or prior to acharging cycle). In order to more rapidly heat the cells 200, the airsupplied by the air supply 30 may be heated to enhance the heating rate.

At optional step S5, the direction of airflow through the air channel 21is reversed so that the air supply 30 forces air through the air outlet17, through the air channel 21 and out of the air inlet 16. For example,the direction of rotation of a fan of the air supply 30 may be reversedto reverse the direction of airflow. This reversal of the airflow may beperformed periodically, either under manual control or by a suitablyconfigured controller of the air supply 30 or aircraft 10, in order toreduce the formation of thermal gradients along the length of the airchannel 21.

At optional step S6, a liquid is flowed through one or more additionalchannels in thermal contact with the cells 200 so that heat is exchangedbetween the cells 200 and the liquid. In this way, supplemental coolingor heating of the cells 200 can be provided. In some embodiments theliquid is pressurized by an on-board pump and flowed through a closedloop contained within the aircraft 10, a portion of which includes theone or more additional channels that are in thermal contact with thecells 200. In other embodiments, the one or more additional channels areconnected to an external supply of liquid which is pumped through theone or more additional channels. The liquid may be cooled or heated byan on-board heater or cooler (e.g., a heat exchanger) or the like inorder to maintain efficient supplemental cooling or heating of thecells.

In order to reduce the weight of the aircraft during flight, theadditional channels may be selectively filled and drained of the liquid.To facilitate this, the aircraft 10 may include a suitable inlet/outletfor filling and draining the liquid as required.

FIG. 10

FIG. 10 illustrates another embodiment of an aircraft 10 and batterypack 20 in which the on-board battery thermal regulation system hasmultiple modes of operation.

As in each of the embodiments described above, the aircraft 10 andbattery pack 20 define an air channel 21 which is in fluid communicationwith one or more aircraft air inlets 16 and air outlets 17. Duringflight, the flow of air through the air channel 21 that results from themovement of the aircraft through the air is leveraged to providecooling. This mode of operation is hereafter referred to as the firstmode of operation of the battery thermal regulation system of FIG. 10 .

Whilst the first mode of operation may provide a high level of coolingduring normal flight, it may be unsuitable for thermally regulating thebattery when there is no or limited airflow through the air channel 21.This will typically be the case when the aircraft 10 is on the ground,but could also be the case during vertical take-off and landingmanoeuvers; where ambient conditions are particularly hot; or where, forexample, the air inlet 16 becomes blocked. To this end, the thermalregulation system of FIG. 10 has a second mode of operation in which anon-board airflow device 40, for example fan, compressor of the like,drives a flow of air through the air channel 21 to thermally regulatethe cells 200.

In the embodiment illustrated in FIG. 10 , the airflow device 40 isarranged within a second air channel 41 separate to the first, main airchannel 21. In the first mode of operation the airflow device 40 doesnot operate and the second air channel 41 may be completely disconnectedfrom the first channel 21. In the second mode of operation, however, thesecond air channel 41 is connected to the first air channel 21 and theairflow device 40 operates to blow a flow of air through the second airchannel 41 and into the first air channel 21.

In some embodiments, the first air channel 21 is completely fluidlydisconnected from the inlet 16 and the outlet 17 in the second mode ofoperation. In this case the first air channel 21 and the second airchannel 41 form a closed loop, and the airflow device 40 circulates airaround the closed loop. The disconnection of the first air channel 21from the inlet 16 and outlet 17 may be achieved by closing the airflowregulators 18, 19, or by using suitable actuators to reconfigure aportion of the airflow path so that air can flow from the second airchannel 41 to the first air channel 21, but air entering the inlet 16cannot reach the first channel 21.

In other embodiments, however, the first air channel 21 may remain influid communication with the inlet 16 and outlet 17 during the secondmode of operation. In this case, the airflow device 40 enhances theambient airflow through the first channel 21. In some embodiments theremay be no second channel at all and the airflow device 40 may instead bearranged within the first air channel 21, or in the path between theinlet 16 and air channel 21. However, this may be less preferred as itmay not be operable where the air inlet 16 becomes blocked. It may alsoconstrain the size or power of the airflow device 40 to the extent thatthe flow rate through the first air channel 21 in the second mode ofoperation is limited.

In preferred embodiments, the airflow device 40 is reversible such thatthe direction of airflow driven through the first channel 21 can bereversed. This may be particularly useful during ground-based charging,where periodic reversal of the airflow direction helps prevent theestablishment of thermal gradients along the length of the first airchannel 21. The direction of airflow can be reversed by, for example,reversing the direction of rotation of a fan or compressor of theairflow device 40 or by reconfiguring the flow path of the secondchannel 41 so that air enters the first channel 21 at the opposite end.A controller (not shown) located in the aircraft 10 or battery pack 20may control the airflow device 40 to periodically reverse the directionof the airflow.

In order to enhance the thermal regulation of the cells 200 in thesecond mode of operation, the airflow driven by the airflow device 40may be cooled or heated as appropriate. For example, the airflow device40 may include or be operably coupled to a heater or cooler (e.g., aheat exchanger or the like) to heat or cool the air. Where present, theheat exchanger may be arranged to dump heat, or receive heat from,outside of the aircraft 10. Alternatively, heat may be dumped to ordrawn from the cabin of the aircraft to provide environmental control.

Although the provision of the on-board airflow device 40 and itsassociated second mode of operation may eliminate the need to use anexternal air supply 30 during ground-based operations, it will beunderstood that an external air supply 30 may still be used with theembodiment of FIG. 10 . Indeed, where an external air supply 30 isavailable, it may be preferable to use it as it may be capable ofproviding more rapid cooling or heating and reduces the amount ofon-board power consumed by the thermal regulation system. However, whereno external supply 30 is available, the second mode of operationprovides an alternative and may permit more rapid charging and thus morerapid turnaround than would otherwise be possible.

FIG. 10 also illustrates additional cooling channels 51, which have beendescribed above with reference to FIGS. 6A-B and 9A-B and are arrangedto carry a liquid in thermal contact with the battery cells 200. Thethermal regulation provided by the additional cooling channels 51 cansupplement or replace the thermal regulation provided by the firstcooling channel 21 during the first and/or second modes of operation.

In this embodiment, the liquid is carried in a closed loop andpressurized by an on-board pressurization unit 50 (e.g., a suitablepump). Preferably the pump is reversible so that the direction of fluidflow through the portion of the channels 51 in thermal contact with thecells 200 can be reversed to eliminate or reduce the formation ofthermal gradients along the length of the pack 20.

If present, the additional channels 51 are preferably arranged so that,during their intended use (e.g., during horizontal flight or on theground), they form an angle with horizontal to assist with degassing. Insome embodiments the aircraft 10 and/or battery pack 20 may be providedwith a liquid inlet/outlet so that the channels can be filled with anddrained of liquid. In this way, the aircraft 10 need not carry theweight of the liquid during flight, but can benefit from additionalground-based thermal regulation, for example to permit more rapidcharging.

Various examples have been described, each of which feature variouscombinations of features. It will be appreciated by those skilled in theart that, except where clearly mutually exclusive, any of the featuresmay be employed separately or in combination with any other features andthe invention extends to and includes all combinations andsub-combinations of one or more features described herein.

1. An aircraft comprising: an aircraft air inlet and an aircraft airoutlet; a battery pack comprising battery cells; and a battery thermalregulation system comprising an airflow device for driving an airflow;and a first air channel, a surface of which is in thermal communicationwith the battery cells so that air flowing through the first air channelexchanges heat with the cells through the surface, wherein: the batterythermal regulation system has a first mode of operation and a secondmode operation; in the first mode of operation, the first air channel isfluidly connected with the aircraft air inlet and the aircraft airoutlet whereby, during flight of the aircraft, air can enter theaircraft through the air inlet, flow through the first air channel andexit the aircraft through the air outlet; and in the second mode ofoperation, the airflow device drives an airflow through the first airchannel.
 2. The aircraft of claim 1, in which the battery thermalregulation system further comprises a second air channel and the airflowdevice is arranged in the second air channel, wherein, in the secondmode of operation, the first air channel is in fluid communication withthe second air channel and the airflow device drives an airflow throughthe second air channel and through the first air channel.
 3. Theaircraft of claim 2, in which, in the second mode of operation, thefirst air channel is disconnected from the aircraft air inlet and theaircraft air outlet and the first and second air channels form a closedloop.
 4. The aircraft of claim 3, further comprising one or moreactuators for disconnecting the first air channel from the aircraft airinlet and the aircraft air outlet and for connecting the first airchannel to the second air channel.
 5. The aircraft of claim 1, in whichthe airflow device is reversible, whereby the direction of the airflowdriven through the first air channel by the airflow device isreversible.
 6. The aircraft of claim 5, further comprising a controllerconfigured to control the reversible airflow device so that, in thesecond mode of operation, the direction of the airflow through the firstair channel is periodically reversed.
 7. The aircraft of claim 1, inwhich the battery thermal regulation system further comprises a coolerfor cooling the airflow driven by the airflow device through the firstchannel.
 8. The aircraft of claim 1, in which the battery thermalregulation system further comprises a heater for heating the airflowdriven by the airflow device through the first channel.
 9. The aircraftof claim 1, further comprising an adjustable airflow regulator forregulating the airflow through the first air channel.
 10. The aircraftof claim 1, in which the first air channel comprises a plurality ofparallel air channels.
 11. The aircraft of claim 1, further comprisingone or more additional channels arranged to carry a flow of liquid in aclosed loop, wherein a portion of the loop is in thermal communicationwith the battery cells so that liquid flowing through the one or moreadditional channels exchanges heat with the cells.
 12. The aircraft ofclaim 11, in which at a portion of the loop of the one or moreadditional channels follow a serpentine path.
 13. The aircraft of claim11, in which the one or more additional channels are arranged so thatthe one or more additional channels form a non-zero angle with thehorizontal.
 14. The aircraft of claim 11, in which the first airchannel, or each of the plurality of parallel air channels of the of thefirst air channel, has a larger cross-sectional area than each of theone or more additional channels.
 15. The aircraft of claim 11, furthercomprising means for draining the one or more additional channels ofliquid.
 16. The aircraft of claim 1, in which the air inlet and the airoutlet are: located in a fuselage of the aircraft; or located in apropulsor of the aircraft.
 17. The aircraft of claim 1, in which thefirst air channel is an integral part of the battery pack.
 18. Theaircraft of claim 1, being of the VTOL type.
 19. The battery thermalregulation system of claim
 1. 20. The combination of the battery packand the thermal regulation system of claim 1.